This invention pertains to a switching amplifier circuit for controlling an inductive load, and in particular, to such a circuit which applies, selectively and reversibly, voltages of opposite polarity to such a load in response to a control voltage indicative of the desired average load current.
A circuit made in conformance with this invention provides an adjustable average current to an inductive load, such as a stepping motor. In operating such a motor, a DC voltage is applied, successively and cyclically, to each of the motor windings. A stepping motor winding can be viewed as an inductive load having a resistance. When a DC voltage is applied to such a winding, the current ramps reverse exponentially in a direction corresponding to the applied voltage. In many conventional circuits, the voltage source is turned off, once a desired maximum current has been attained, until the current falls to a level below the desired current, at which time it is turned on again. This cyclical operation maintains essentially a constant average current level in the winding. Alternatively, a winding may be center-tapped, which tap is either connected to a fixed voltage source or to ground. The opposite ends of the winding are then alternatively connected, appropriately, to the power supply or ground through a switch such as a transistor.
Additionally, the feedback for current-source-switching amplifiers has been accomplished conventionally by the use of resistive dividers and operational amplifiers. These components inherently limit the feedback response time and hence switching rate. Another circuit known to the applicant uses a pulse-controlled Schmitt trigger to control the conductivity of a transistor switch connecting a winding terminal to ground.
These prior art techniques tend to have significant ground return current. Also, these circuits do not provide for adjusting the dither current range, and therefore, frequency, above and below the average current level.
It is therefore a general object of this invention to overcome the above-described disadvantages of the prior art techniques.
It is specifically an object to provide a fast-switching amplifier circuit, responsive to an external voltage signal, for adjustably controlling the average current of an inductive load.
It is also an object to provide such a circuit having a current-sensing circuit with a common mode voltage near zero, and having both minimum circuit ground return current levels and minimum grounding noise.
A further object is to include adjustable hysteresis circuitry in the amplifier circuit for varying the magnitude and as a result, the frequency, of the load current relative to a desired average current.
An object of the present invention is also to provide a circuit which permits the doubling of the drive voltage across the load impedence as compared to normal ground return drives, thereby permitting the doubling of available slew rate capability.
In the preferred embodiment of the present invention, an essentially balanced bridge network is connected to terminals which are connectable to an inductive load. This network contains a resistance in series with each terminal. Cross-over resistance paths are also provided from the load side of each in-line resistance to the non-load side of the other in-line resistance. A voltage is taken between intermediate taps disposed in the cross-over resistance lines to indicate the current level in the load. A differential voltage existing at a comparator input has components derived from this load-current-indicating voltage and a control voltage to produce a pair of digital binary outputs which are complementary with respect to each other. A source connects, selectively and reversibly, power supply voltages of equal magnitude and opposite polarity to the load terminals in response to the digital outputs of the comparator. Hysteresis circuitry is connected to the comparator to cause it to switch when the load current varies from the desired load current by a known range.
With the bridge network and applied terminal voltages balanced about a ground or zero level, there is a near zero common mode current-sensing feedback voltage and near zero ground return current. By making the comparator responsive to an input control voltage, the load current may be adjusted for operation at different levels. The hysteresis also provides for operating the switching amplifier circuit at a frequency which is appropriate for a particular load.
These and additional objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following detailed description of the preferred embodiment.